Many of today's electronic circuits require highly accurate voltage or current references in order to function within stringent specification requirements. These references provide either a supply independent and/or temperature independent current or voltage, which allow the entire circuit to function properly under a wide range of the external supply voltages and temperatures. Consider electronic sensors used to measure a physical quantity like pressure or acceleration. It is required that the measurement of the carrying information output will be within some predetermined error band. This implies that sensor's signal conditioning circuit must be implemented in a way that meets the required output accuracy. To accomplish this, among other things, some sort of accurate reference is needed, for example a biasing current. It is not unusual that this reference current has to be accurate to +/-30 parts per million per degree Celsius (ppm/.degree.C.).
Also, as the technology of integrated circuits advances, minimizing the size of the IC and keeping any external parts needed to a minimum is of primary importance.
Some prior art solutions have used a precision reference voltage such as a bandgap circuit embodied on the IC and an external, low temperature coefficient (TC) resistor, to generate a precision current. When referring to a low TC resistor, it is meant that the TC of the resistor is on the order of magnitude of +/-30 ppm/.degree.C. The integrated bandgap circuit acts as a source of a supply independent and temperature compensated voltage. While this solution accomplishes the goal of providing a precision current reference which is temperature and supply independent, the need for the external low TC resistor takes up valuable board space and significantly increases the cost and decreases the reliability of the circuit.
Other prior art solutions use a bandgap voltage reference and a special internal, integrated resistor that has the lowest possible TC. However, the lowest possible TC resistor in, for example, a typical CMOS IC process is a special buried n-type resistor with a TC of approximately 320 ppm/.degree.C. Because the TC of this resistor is non-zero, the resultant current reference does not generate a temperature independent current reference and will have a TC of -320 ppm/.degree.C. when sourced with a zero TC bandgap circuit voltage. Thus, this solution does not accomplish the goal of providing a precision current reference which is temperature independent even though supply independence is achieved. Therefore, it would be highly desirable to provide a temperature and supply independent current reference fully contained on the IC and without the need for any external resistors.